A power supply step-down circuit is adapted to a semiconductor integrated circuit having a first operation mode and a second operation mode having a smaller current consumption than the first operation mode. The power supply step-down circuit includes a first step-down circuit activated only during the first operation mode to step down an input power supply voltage to an output voltage, a second step-down circuit provided integrally with the first step-down circuit and activated only during the second operation mode to step down the input power supply voltage to an output voltage, an output terminal to output the output voltage of one of the first and second step-down circuits that is activated, and an output circuit to maintain the output voltage that is output from the output terminal lower than the input power supply voltage for a first predetermined time when an operation mode makes a transition from the first operation mode to the second operation mode.